The present invention relates to a catalyst composition for use on a diesel particulate filter, and more particularly, to a catalyst composition which effectively facilitates the oxidation of soot without generating increased NO2 emissions during regeneration of the filter.
In recent years, environmental regulations in the United States and Europe restricting diesel particulate emissions have necessitated improvements in the removal of particulates from diesel engine emissions. Such particulates generally consist of carbonaceous particulates in the form of soot. Currently, the most commonly used method for removing soot from engine exhaust is the use of a diesel particulate filter (“DPF”) which collects the soot, followed by oxidation of the accumulated particulates at elevated temperatures which regenerates the filter.
However, a problem which occurs during DPF regeneration is the increase in nitrogen dioxide (NO2) emissions. While the specific emission of NO2 during regeneration is not currently regulated by legislation, such NO2 emissions are still a health concern as such emissions can be harmful to the lungs. For example, if DPF regeneration is carried out in an enclosed area such as a garage, a huge increase of pollutants may occur in the air surrounding the vehicle. In some instances, these increased emissions may lead to high pollutant concentrations which exceed occupational health standards.
The problem of increased NO2 production during the particulate filter regeneration process is related to the mechanism of soot combustion in the particulate filters. Several different methods of regenerating particulate filters are known. One method uses a non-catalyzed diesel particulate filter. A problem with the use of such non-catalyzed diesel particulate filters is that regeneration of the filter by soot oxidation can be difficult due to the low temperatures of diesel exhaust gases (e.g., less than about 200° C.), which are unfavorable for soot oxidation. Accordingly, it is necessary to use periodic high-temperature regeneration of the filter to oxidize the soot at elevated temperatures.
Another known particulate filter system relies on fuel-borne catalytic assistance in the regeneration of the DPF, i.e., the inclusion of a catalyst, typically metal, in the fuel as an additive which functions to lower the temperature at which carbon combusts. However, such a system is complex and requires additional components such as a tank for fuel additives, an additive dosing system, and infrastructure to refill the additive fuel tank. In addition, the use of fuel-borne catalysts can lead to the formation of ash which accumulates on the filter, causing gradual loss of filter soot capacity and a decrease in time between regeneration events. Therefore, it is necessary to change the filter after about every 80 K kilometers.
Another known method for removing soot is to deposit a catalyst on the walls of the DPF, also referred to as a “catalyzed DPF” or “CDPF.” A catalyzed soot filter typically comprises one or more platinum group metal catalysts and/or a palladium containing oxidation catalyst to improve soot oxidation on the filter. The use of catalyzed diesel particulate filters are generally preferred for use in soot oxidation during regeneration due to the fact that they are less complex than fuel-assisted diesel particulate filters, and achieve effective soot oxidation at lower temperatures relative to non-catalyzed, uncoated filters. However, such catalyzed soot filters are typically expensive to produce due to the high cost of platinum group metals.
In addition, the amount of NO2 emissions generated from these known methods of diesel particulate filter regeneration varies. Typically, regeneration methods which utilize non-catalyzed diesel particulate filters and fuel-borne catalyst assisted particulate filters demonstrate a low NO2 production ratio (i.e., the amount of NO2 at the inlet of the filter is higher than the NO2 emissions at the outlet). The exhaust gas downstream of the non-catalyzed or fuel-borne catalyst assisted diesel particulate filter has a lower NO2 concentration in comparison with the inlet concentration due to the reaction of NO2 with soot during regeneration:C+NO2→CO+NO.
On the other hand, conventional catalyzed diesel particulate filters which use platinum-based catalysts show a high NO2 production ratio, i.e., the amount of NO2 on the filter outlet is higher than the NO2 upstream of the filter inlet, indicating an increase in the production of NO2 during regeneration.
As the use of catalyzed diesel particulate filters are generally preferred for use in soot oxidation, it would be desirable to be able to decrease NO2 emissions generated during filter regeneration to the low level achieved with other types of filters while still maintaining the soot combustion properties. It would also be desirable to eliminate all or part of the platinum group metals used in such filters.
While attempts have been made to suppress or reduce the amount of NO2 emissions in diesel particulate filters in general (i.e., with the use of a reduction catalyst such as urea, fuel, or hydrocarbons placed in the exhaust gas flow path), no solution has been developed for decreasing NO2 emissions during regeneration without the use of a reduction catalyst.
Accordingly, there is still a need in the art for a catalyst composition for use in a diesel particulate filter which can effectively oxidize soot during periodic high temperature regenerations, which uses little or no platinum, and which effectively decreases NO2 emissions produced during such regenerations without the need for a reductant or other additional equipment.